I made some calculations about the UA175 output stage and transformer, and I don't know if I got everything right, so I better ask on th eforum before I go shopping.
Problem is, I don't know what output transformer type was used, and calculating backwards gives some surprising (to me!) results.
Here's the outline of the math:
Output stage is 12BH7 pp with ca. 300V plate voltage.
Each triode has a bias current of ca. 17mA, both triodes sharing a 330R cathode resistor.
Transformer output 600R.
Calculating winding ratio / primary impedance:
Manual says level at the onset of limiting is +13dBm.
That would be 3,5Veff or 5Vpeak on the seconday.
On the primary, the diodes start to conduct at ca. 45Vp.
I guess this must be the onset limiting.
Calculating 45V/5V = 9:1 for one side, so the transformer
would be
18(c.t.) : 1
200kOhm : 600 Ohm
This looks like a rather high ratio to me - I had expected something
around 5, but little do I know.
Is the 200kOhm p-p value reasonable?
Of course this means almost no load to the triodes running at high current.
An almost horizontal load line ?!
Then the next info from th emanual is the point of clipping when limiting is turned off: +24dB.
That would be 12Veff or 17Vp on the secondary, and 153Vp (306Vpp) at *one* plate of the output tube.
Can the 12BH7 go down to 140V? Spice simulation says no, but I'm pretty sure that Spice is wrong here. The triode shoud go down that far with no problem, given that light load, shouldn't it? And the whole pp stage should even stay in class A all the time, shouldn't it?
(I really don't know what's wrong here. Spice simulated the WCF in my LA2A-based limiter quite ok. Also using 12BH7 models.)
That's what I have cobbled together so far. Please give me some confirmation or correction before I go shopping for a 18:1 transformer.
Hints where to buy such a transformer are welcome, too. (Must be 250mW - 24dBm - or higher).
Hints about the *original* transformer would also be appreciated!
JH.
Problem is, I don't know what output transformer type was used, and calculating backwards gives some surprising (to me!) results.
Here's the outline of the math:
Output stage is 12BH7 pp with ca. 300V plate voltage.
Each triode has a bias current of ca. 17mA, both triodes sharing a 330R cathode resistor.
Transformer output 600R.
Calculating winding ratio / primary impedance:
Manual says level at the onset of limiting is +13dBm.
That would be 3,5Veff or 5Vpeak on the seconday.
On the primary, the diodes start to conduct at ca. 45Vp.
I guess this must be the onset limiting.
Calculating 45V/5V = 9:1 for one side, so the transformer
would be
18(c.t.) : 1
200kOhm : 600 Ohm
This looks like a rather high ratio to me - I had expected something
around 5, but little do I know.
Is the 200kOhm p-p value reasonable?
Of course this means almost no load to the triodes running at high current.
An almost horizontal load line ?!
Then the next info from th emanual is the point of clipping when limiting is turned off: +24dB.
That would be 12Veff or 17Vp on the secondary, and 153Vp (306Vpp) at *one* plate of the output tube.
Can the 12BH7 go down to 140V? Spice simulation says no, but I'm pretty sure that Spice is wrong here. The triode shoud go down that far with no problem, given that light load, shouldn't it? And the whole pp stage should even stay in class A all the time, shouldn't it?
(I really don't know what's wrong here. Spice simulated the WCF in my LA2A-based limiter quite ok. Also using 12BH7 models.)
That's what I have cobbled together so far. Please give me some confirmation or correction before I go shopping for a 18:1 transformer.
Hints where to buy such a transformer are welcome, too. (Must be 250mW - 24dBm - or higher).
Hints about the *original* transformer would also be appreciated!
JH.